Embodiments of the inventive concept relate generally to semiconductor memory devices. More particularly, embodiments of the inventive concept relate to semiconductor memory devices having a stacked structure and related methods of manufacture.
Semiconductor memory devices can be roughly divided into two categories according to whether they retain stored data when disconnected from power. These categories include volatile memory devices, which lose stored data when disconnected from power, and nonvolatile memory devices, which retain stored data when disconnected from power. Examples of volatile memory devices include dynamic random access memory (DRAM) and static random access memory (SRAM). Examples of nonvolatile memory include phase-change RAM (PRAM), resistive RAM (RRAM), magnetic RAM (MRAM), and electrically erasable programmable read only memory (EEPROM).
Nonvolatile memory devices have gained increasing popularity in recent years due to a variety of trends, such as the increasing demand for long term data storage and the proliferation of mobile devices that need to retain stored data when disconnected from power.
Some nonvolatile memory devices store data using variable resistance elements that change between different resistive states to represent different data values. Such nonvolatile memory devices are referred to as resistive memory devices. A resistive memory device typically comprises a plurality of memory cells each comprising a variable resistive element and a switching element. The variable resistive element is connected between a bitline and the switching element, and the switching element is typically connected between the variable resistive element and a wordline. The memory cells are generally connected in a memory cell array connected to a plurality of wordlines and bitlines.
Examples of resistive memory devices include PRAM, RRAM, and MRAM. In a PRAM, the variable resistance element comprises a phase-change material such as chalcogenide that changes resistance in response to an applied temperature. In an RRAM, the variable resistance element comprises an upper electrode, a lower electrode, and a transition metal oxide between the upper and lower electrodes. In an MRAM, the variable resistive element comprises a ferromagnetic upper electrode, a ferromagnetic lower electrode, and a ferromagnetic material between the upper and lower electrodes. In an effort to improve the performance and storage density of resistive memory devices, researchers have developed resistive memory devices in which memory cell array layers are stacked in a 3-dimensional configuration on a semiconductor substrate.